1. Field of the Invention
The invention relates generally to methods of construction for acoustic and electrically amplified stringed musical instruments. The invention further relates to acoustic and electrically amplified stringed musical instruments comprising fiber-reinforced resin composite materials, where the instruments are provided with a frequency-damping interior coating.
2. Description of the Related Art
Stringed musical instruments, e.g., guitars, mandolins, lutes, violins, cellos, and the like, both acoustic and electrically amplified, have traditionally been constructed of wood. More recently, stringed instruments have been made from wood, molded plastics, molded composite materials, or a combination of wood, plastics and composite materials. The body of the stringed musical instrument may be solid, semi-hollow, or hollow. The neck is typically solid and may further include a truss rod for increased neck strength.
Stringed musical instruments constructed of wood typically have a pleasing resonance but lack the durability of instruments that are made from synthetic composite materials. When composite materials are used or wood and composite materials are combined in the construction of a stringed musical instrument, the durability and strength of the instrument are improved. However, the bulk density of the composite materials is much greater than the density of wood, and the sound-absorbing properties of the composite materials are quite different than those of wood. Hence the resonances of the instrument are changed so that it may produce unpleasing and unacceptable tone characteristics, sometimes described as xe2x80x9cinny.xe2x80x9d
Fiber-reinforced composites are appealing materials of construction for stringed instruments because such materials are light, stiff and far more resistant to environmental variables, particularly moisture and heat, than are the fine woods traditionally used. Composites are also mechanically stronger than other synthetic materials, e.g., molded plastics. For example, U.S. Pat. No. 4,290,336 describes a molded plastic guitar; this guitar has cost advantages but still requires ribs, a torsion rod, etc., to provide sufficient resistance to mechanical stresses. U.S. Pat. No. 4,313,362 discloses a molded plastic guitar; it features a reinforcement rod that runs from the butt end of the body to the upper portion of the peghead. U.S. Pat. No. 4,188,850 discloses a guitar made of metal and foamed plastic; the neck is formed of a metal body with plastic foamed around it. The neck of a stringed instrument is especially subject to warping because of the tension placed on it by the strings, which naturally varies across the strings from high to low. Thus, replacing all or part of the wood with a fiber-reinforced composite material has been a long-sought goal, especially for the neck of the instrument.
While the potential durability and strength of stringed instruments made in whole or in part of fiber-reinforced composite materials are well accepted, the tone qualities emitted by such instruments have not always been appreciated, nor have manufacturing methods that are simple and readily reproducible been available. Manufacturing methods for fiber-reinforced composite articles will preferably employ a minimum number of steps that require cutting, machining and joining. The resulting musical instruments will preferably emit pleasing tones.
The challenge of fabricating fiber-reinforced composite stringed musical instruments that produce pleasing sounds is appreciable. Composite materials, e.g., resins such as epoxy reinforced with fibers such as graphite, boron, or glass, differ greatly from wood in their acoustic damping properties. Wood is very xe2x80x9clossyxe2x80x9dxe2x80x94heavily dampingxe2x80x94in the sonic frequency range, especially in the high frequencies. While there has been extensive study of this topic, for example, Materials Research Society Symposium on Materials in Musical Instruments (1994), published in MRS Bulletin, XX, No. 3 (March 1995), the characteristics of pleasing sound quality are not readily quantified. The state of the art was summarized in quite an interesting way as follows (xe2x80x9cGraphite Guitar Acoustics 101,xe2x80x9d John A. Decker, Jr., Nov. 4, 1999, http://www.rainsong.com/acc101.htm): xe2x80x9cThere have been a number of experiments where people were asked to tell a Stradivarius violin, say, from a junk student fiddle or a Ramirez classical guitar from a junk guitar when they were played behind a curtain. Even xe2x80x9cnaxc3xafvexe2x80x9d subjectsxe2x80x94people off-the-streetxe2x80x94can almost always differentiate the quality instrument from the junk one. However, if one tries to identify them by their frequency spectrum or mode structure, even musical-instrument acoustic physicists who have spent their entire careers working in this field can""t tell which is which.xe2x80x9d
The neck of a stringed musical instrument has been a focus of effort to strengthen and stiffen the instrument. U.S. Pat. No. 4,145,948 discloses a guitar neck constructed from graphite fiber reinforced plastic, in which the graphite fibers are oriented longitudinally to provide high stiffness in the direction of the strings. The neck componentsxe2x80x94the base of the neck, a top piece, and a fingerboardxe2x80x94are molded separately and then adhesively bonded together. No provision for sound frequency damping is disclosed. The inventor of the ""948 patent, in a subsequent patent (U.S. Pat. No. 4,846,038), states that this hollow neck xe2x80x9crequires an inordinate amount of machining and finishing.xe2x80x9d The ""038 patent discloses a solid guitar neck that has a graphite fiber reinforced composite T-bar in the neck body, and an attached fingerboard into which are spiked the frets. U.S. Pat. No. 4,846,039 discloses a solid guitar neck formed from alternating layers of epoxy and powdered carbon, fiber reinforced. This solid neck may be constructed with an integral fingerboard. U.S. Pat. No. 4,950,437 also discloses a fiber-reinforced composite neck that can be constructed with an integral fingerboard, by wrapping resin-impregnated fiber cloth around a neck insert, placing the wrapped insert in a mold, pressing the fingerboard down on the top surface of the wrapped insert, and curing the resin. The neck insert can be removed to produce a hollow neck. No provision is disclosed for damping of high frequencies to improve the quality of the sound emitted. U.S. Pat. No. 6,100,458 discloses a composite neck for a stringed instrument; the neck is molded with resin-impregnated fiber cloth around a foam core.
The soundboard of the stringed musical instrument has also attracted innovative materials approaches. In acoustical instruments, most of the sound quality arises from the soundboard. U.S. Pat. Nos. 4,873,907 and 4,969,381 disclose a fiber-reinforced composite soundboard, with a foam core. In order to avoid the xe2x80x9ctinnyxe2x80x9d sound of soundboards made entirely of graphite-resin composites, a layer of acoustically dead fabric such as Kevlar(copyright) or Dacron(copyright) can be incorporated into the layers of graphite fiber weaves before curing. The resulting bulk density of the composite is 2-4 times that of wood; the composite is made thinner so that the areal density is approximately the same as wood. The soundboard, side and back are made separately, machined and joined. U.S. Pat. No. 5,333,527 discloses an acoustic guitar soundboard composed of compression molded, graphite-reinforced epoxy plastic. The soundboard can be provided with bracing ribs similar to a wooden soundboard in the molding process or afterwards; the sound quality is stated to be capable of being manipulated by forming the soundboard with various curved surfaces. U.S. Pat. No. 6,107,552 discloses a thin and light but strong soundboard fabricated of two outer layers of graphite reinforced sheet material sandwiched around a layer of low-density core material, such as rigid polyvinyl chloride.
A challenge of using synthetic composite materials is adapting the range of sound frequencies produced to be satisfactory to the ear. In some cases the goal is to simulate as closely as possible the sound of a wood instrument; another goal may simply be the production of an inherently pleasant sound regardless of its similarity to the sound of a traditional wood instrument. For example, U.S. Pat. No. 5,905,219 discloses a polyurethane stringed instrument, where the density of the polymer is adjusted by adding various amounts of inorganic filler materials such as glass bubble, etc., with the goal of controlling the sound quality produced by the instrument. In composite materials, carbon fibers are desirable reinforcing fibers because of their high strength-to-weight ratio, their high modulus of elasticity, and their low coefficient of thermal expansion. A problem with achieving a pleasing sound with carbon-fiber-reinforced composites is their lower degree of energy absorption relative to wood, which is very lossy, especially at high frequencies. The lower degree of energy absorption is desirable for sustain qualities and harmonic clarity, yet is undesirable due to the relative excess in high frequencies. In general, some form of damping needed, such as the layer of acoustically dead fabric disclosed by U.S. Pat. Nos. 4,873,907 and 4,969,381 mentioned above. U.S. Pat. No. 4,364,990 discloses a fiber-reinforced composite material suitable for constructing stringed musical instruments, where this damping function is provided by a layer of xe2x80x9ccellulosic materialxe2x80x9dxe2x80x94cardboard or paperxe2x80x94sandwiched between layers of graphite-epoxy prepreg which are then pressed and heated in a mold to permanently bond the mat of cellulose fibers within the cured composite. U.S. Pat. No. 5,895,872 seems to disclose a guitar basically formed from a graphite-epoxy composite, with some commingling of aramid fibers, presumably for sound-damping purposes. U.S. Pat. No. 6,087,568 describes an approach to control the tone qualities of a composite stringed instrument by formulating the composite material to include in addition to the resin carbon fibers, glass fibers, and a sound-damping filler such as glass microballoons.
One factor that motivates use of fiber-reinforced composite materials is the hope of simplifying the construction process. This goal has not been fully achieved. U.S. Pat. No. 5,955,688 discloses a stringed musical instrument exemplified as a violin made from graphite fiber reinforced epoxy resin. The fabrication is still relatively complex, with the body, belly and soundboard separately molded and then joined, whereupon a pair of struts, a soundpost, a bridge, and a string assembly are affixed.
There remains a need for a method for manufacturing fiber-reinforced composite stringed musical instruments, where the method is simple, reproducible, and may be readily scaled for manufacturing, and the resulting instruments emit pleasing musical tones and are light, strong and resistant to environmental degradation.
The present invention in one aspect relates to a stringed musical instrument or part thereof, where the instrument or part thereof is a unitary hollow structure. The stringed musical instrument or part thereof comprises an exterior shell structure formed in a resin matrix fiber-reinforced composite and an interior polymeric sound-damping layer bonded to all or a portion of the interior surface of the exterior shell. The resin matrix may be, for example, an epoxy, polyester, vinylester, or phenolic resin, or other suitable curable resin. The reinforcing fibers are strong and stiff fibers, for example, carbon, boron, silicon carbide, or tungsten; carbon is preferred. In addition to the reinforcing fibers, the composite may also include other light-weight fibers, such as glass fibers, present in separate layers or intermingled. The reinforcing fibers are present in the outermost and innermost layers. The interior sound-damping layer comprises an elastomeric material tightly bonded to all or part of the interior surface of the hollow composite. To enhance its appearance, the exterior surface of the instrument may have decorative features, for example, a molded-in exterior decorative textile layer, inlaid designs, or paint. The neck of the stringed musical instrument may be provided with frets that are adhesively attached to it rather than pounded in as traditional spike-like structures. Optionally, polymerized foam may be present in part or all of the hollow interior of the instrument or part thereof, to modify sound production.
In a preferred embodiment, the hollow stringed musical instrument or part thereof, comprises an exterior shell comprising an epoxy matrix, carbon fiber reinforced composite and an elastomeric sound-damping layer bonded to all or part of the interior surface of the exterior shell.
In another aspect, the invention comprises a process for the unitary molding of a fiber reinforced composite hollow stringed musical instrument or part thereof comprising the steps of:
laying one or more pieces of fiber cloth into each of a top mold and a bottom mold, said top and bottom molds shaped to form respectively top and bottom portions of the hollow stringed musical instrument or part thereof, where the cloth pieces are sized so that cloth extends beyond the edges of the top and bottom molds, and where the cloth pieces are optionally impregnated with a heat-curable resin (prepreg);
adding heat-curable resin as needed to the cloth in an amount sufficient to fully saturate the cloth;
mating the top and bottom molds with an inflatable polymeric balloon inserted therebetween;
inflating the polymeric balloon to a pressure sufficient to press the cloth pieces firmly against the molds; and
heating the mated molds to a curing temperature for a curing time sufficient to cure the heat-curable resin.
In preferred embodiments, the resin matrix is selected from epoxy, polyester, vinylester, or phenolic resins and the reinforcing fibers are selected from carbon, boron, silicon carbide, and tungsten fibers. The process may further comprise placing one or more additional layers of fiber cloth into each of the top and bottom molds, where the additional layers of fiber cloth may contain the reinforcing fibers, glass fibers, or a mixture of both fiber types. In preferred embodiments, carbon-fiber-containing cloth and glass-fiber-containing cloth are laid in the molds in 2 to 20 layers, with the carbon-fiber-containing cloth being used for the first and last cloth layers. In especially preferred embodiments, the carbon-fiber-containing cloth and glass-fiber-containing cloth are laid in the molds in such a manner as to create alternating carbon-fiber-reinforced and glass-fiber-reinforced layers in the hollow cured composite structure, with carbon-fiber-reinforced layers forming the exterior surface and the interior surface which is bonded fully or partially to an elastomeric sound-damping layer.
In a further aspect of the invention, the process for the unitary molding of a hollow stringed musical instrument or part thereof comprising the steps of:
laying one or more pieces of fiber cloth into each of a top mold and a bottom mold, said top and bottom molds shaped to form respectively the top and bottom halves of the hollow stringed musical instrument or part thereof, where the cloth pieces are sized so that cloth extends beyond the edges of the top and bottom molds, and where the cloth pieces are optionally impregnated with a heat-curable resin;
adding heat-curable resin as needed to the cloth in an amount sufficient to fully saturate the cloth;
mating the top and bottom molds;
curing the heat-curable resin by a vacuum molding process; and
coating an interior surface of the hollow stringed musical instrument or part thereof with a polymeric sound-absorbing coating. This material is typically an elastomer such as rubber, silicone, etc.
In preferred embodiments, the resin matrix is selected from epoxy, polyester, vinylester, or phenolic resins and the reinforcing fibers are selected from carbon, boron, silicon carbide, and tungsten fibers. The process may further comprise placing one or more additional layers of fiber cloth into each of the top and bottom molds, where the additional layers of fiber cloth may contain the reinforcing fibers, glass fibers, or a mixture of both fiber types. In preferred embodiments, carbon-fiber-containing cloth and glass-fiber-containing cloth are laid in the molds in 2 to 20 layers, with the carbon-fiber-containing cloth being used for the first and last cloth layers. In especially preferred embodiments, the carbon-fiber-containing cloth and glassfiber-containing cloth are laid in the molds in such a manner as to create alternating carbon-fiber-reinforced and glass-fiber-reinforced layers in the hollow cured composite structure, with carbon-fiber-reinforced layers forming the interior and exterior surfaces.
The process of fabricating the instrument may also include providing the exterior surface of the instrument with decorative features, for example, a molded-in exterior decorative textile layer, inlaid designs, or paint. The neck of the stringed musical instrument may be provided with frets that are adhesively attached to it rather than pounded in as traditional spike-like structures. The instrument may be provided with magnetic or piezoelectric pick-ups for sound amplification.